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Article: Kamani, Mohammad Hassan, Eş, Ismail, Lorenzo, Jose M. et al. (5 more authors) (2019) Advances in plant materials, food by-products, and algae conversion into biofuels : Use of environmentally friendly technologies. Green Chemistry. pp. 3213-3231. ISSN 1463-9262 https://doi.org/10.1039/c8gc03860k

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Advances in plant materials, food by-products, and algae conversion into biofuels: Use of environment-friendly technologies zReceived 00th Mohammad Hassan Kamani1,a, Ismail Eş1,b, Jose M. Lorenzoc, Fabienne Remized, Elena January 20xx, Roselló-Sotoe, Francisco J. Barbae, James Clarkf, Amin Mousavi Khaneghahg,* Accepted 00th January 20xx Green technologies have emerged as useful tools for the generation of clean fuels with the potential to minimize the effect of human activity on the environment. Currently, these fuels DOI: are mainly composed of hydrocarbons obtained from crude oil. Over the last two decades, 10.1039/x0xx00000x has gained significant attention as a renewable feedstock for more sustainable www.rsc.org/ production and has been a great candidate to replace fossil fuels. Principal components of most of the available biomass are cellulose, hemi-cellulose, and lignin. Although available green technologies for biofuel production are progressing rapidly, productivity and chemical yield from these techniques are still below the required values. Therefore, there is a need for interdisciplinary studies to meet the requirements for more global and efficient production by streamlining processes, integrating technologies and achieving techno-economic improvements. In this context, we aim to give an overview of available biomass such as agricultural wastes suitable for the generation of different classes of biofuels including next-generation biofuels. Unfortunately, expensive, wasteful and

Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. energy-consuming pretreatment processes are still used. Therefore, novel technologies that allow a more efficient a. Young Researchers and Elite Club, Sabzevar Branch, Islamic Azad University, Sabzevar, Iran b. Department of Material and Bioprocess Engineering, School of Chemical separation with low resource consumption and the Engineering, University of Campinas (UNICAMP), Campinas, São Paulo, Brazil generation of a low number of residues are required. In c. Centro Tecnológico de la Carne de Galicia, rúa Galicia n° 4, Parque Tecnológico de Galicia, San CibraodasViñas, Ourense, Spain this regard, the novel technologies such as efficient d. UMR QualiSud, Université de La Réunion, CIRAD, Université Montpellier, Montpellier SupAgro, Université d'Avignon, Sainte Clotilde, France fractionation techniques, genetic and metabolic e. Nutrition and Food Science Area, Preventive Medicine and Public Health, Food Science, Toxicology and Forensic Medicine Department, Faculty of

Pharmacy, Universitat de València, Avda. Vicent Andrés Estellés, s/n, engineering including the application of CRISPR/Cas tools, Green Chemistry Accepted Manuscript 46100 Burjassot, València, Spain f. Green Chemistry Centre of Excellence, Department of Chemistry, as well as microfluidic platforms to improve the overall University of York, Heslington, York, YO10 5DD, United Kingdom g. Department of Food Science, Faculty of Food Engineering, University of yield of biofuel production are discussed. Campinas (UNICAMP), Rua Monteiro Lobato, 80. Caixa Postal: 6121.CEP: 13083-862. Campinas. São Paulo. Brazil.E-mail: [email protected] h. 1M H Kamani and Ismail Eş contributed equally in this work † Footnotes relating to the title and/or authors should appear here. Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x

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Introduction They can be solid, liquid or gaseous fuels,View which Article Online are DOI: 10.1039/C8GC03860K Sustainability is a key-element for worldwide economic produced from biomass and can be used either purely or 2,3 development. Energy is an essential part of people's daily as blended forms with other fuel types . life. There are diverse energy sources such as natural gas, Biofuels have many benefits which are: (a) availability coal, and oil which can be utilized for the production of from existing biomass sources, (b) environmentally fuel, heat, electricity etc1. With increasing world friendly potential and lower threat to the ecosystem, (c) population, the demand for diverse types of fuels has biodegradability, renewability and contribution to sharply risen mainly due to industrialization and sustainability, (d) beneficial for the economy including motorization over the world2–4. This excessive extending the opportunities for agriculture, (e) increasing consumption has generated a high concentration of industrial investment, boosting agricultural income, atmospheric pollution and, in particular, a steep increase creating rural manufacturing jobs and (f) achieving energy 2,4,8–10 in the amount of greenhouse gasses including carbon security . Due to all these advantages, biofuels are dioxide, sulfur dioxide, and nitrogen oxides from the becoming competitive with fossil fuels, and are forecast to 2 burning of fossil fuels1,3,5. grow even faster in the next decade . Therefore, the This has likely led to many adverse consequences major goal of this review is to provide a detailed notably changes in climate, loss of glaciers, the rise in discussion on definition, reaction pathways, agricultural global sea level, and loss of biodiversity. On the other sources, production method (conventional and innovative hand, fossil fuels have limited sources which are being green techniques), and existing challenges with common exhausted due to overconsumption per capita4,6. forms of liquid and gas biofuels.

Therefore, most countries have been revising their Biofuels production policies and shifting their focus towards clean and Agricultural wastes as a biomass source for biofuels renewable fuels to meet their future demands1. In this respect, the Kyoto protocol ratified the target of The major source of biofuel is biomass and, for this 3,10 decreasing carbon dioxide concentration through reason, biofuel is also called biomass-based fuel . Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. reduction of dependency on fossil fuels1,5. Biomass is defined as an organic substance, which has 9 The scientific community has made a lot of efforts to stored sunlight in the form of energy by photosynthesis . employ versatile sources and emerging technologies for It refers to any renewable type of plant-based material developing renewable fuels which are more cost- which can be used for the production of energy, like 1,10,11 effective, efficient and sustainable with less emissions4,5,7. transport fuel, power or heat . It is considered to be a Among all energy sources, biomass has gained particular relatively attractive feedstock because of a) renewability,

Green Chemistry Accepted Manuscript attention due to its numerous advantages over fossil b) positive environmental properties (lower release of resources2,4. It is a favorable source for production of carbon dioxide and sulfur content than fossil energy) and clean energies like biofuels. Biofuels are being explored as c) significant economic potential when compared to fossil 2,6,11 an attractive choice for addressing these crises i.e., resources . Biomass can be converted into biofuels 5 reliance on fossil source and greenhouses gas emissions4. using different thermal, physical or biological processes . There are various categories of plant-based biomass,

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which fall into two main groups including agricultural wide varieties of terpenes such as α-pineneView, β- Articlepinene, Online DOI: 10.1039/C8GC03860K biomass (e.g., grasses, from and , stalks, camphene, limonene, 3-carene, 1,8-cineole, spathulenol, bran/husk, crop residue, seeds and plant food by- myrcene etc. It has been reported that global industry products), and forestry resources (e.g., thinned wood and recovers 3 million tons of these hydrocarbons per year. sawdust, logging residues and leaves). However the non- However, some of them (such as β-pinene or myrcene) plant-based biomass are classified into livestock resources have been identified to meet current chemical and (e.g., butchery waste), fishery sources (e.g., industrial industrial requirements (e.g., viscosities, freezing and fishery processing by-products), industrial biomass flash points and density), and therefore, have potential to (sewage sludge), household biomass (e.g., garbage waste) be used directly or blended with existing fuel like jet fuel and plantation sources (e.g., aquatic algae, (e.g., JP-5, Jet-A, and JP-8), gasoline, or other types photosynthetic organisms)9,12,13. Fig. 1 shows the diesels15. potential agro-residues, which can be used as biomass for Therefore, in view of the importance and capability of biofuels production3,4,7,14. above-mentioned resources, utilizing these potential Among these categories, agricultural waste conversion plant/agro-resources for fuel production in an appropriate represents approximately 64% of the total energy way is highly necessary providing the “double green” demand and has the most significant contribution to benefits of avoiding uncontrolled release of pollutants biomass energy9. Agricultural waste refers to the residues into the atmosphere and substituting non-renewable produced in fields or on farms during harvesting and fossil fuels. other activities9,13. Many developing countries have a Biofuel classification wide range of agricultural wastes in abundant quantities, which are regularly disposed instead of being used as Biofuels can be broadly classified based by the type and biomass source9. For instance, rice straw is globally sources of biomass, e.g., residues from agriculture, food produced at around 600-900 million tons per year. Only a industry, fishery or municipal wastes4. Biofuels can be also small portion of this straw is directly used (as animal categorized according to primary or secondary generation Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. feed), and the remainder is mostly burnt from the field1. or based on their forms and applications (Table 1)4. First Another example is corn straw, where more than 90% is generation biofuels were produced without processing left in the fields in the United States1. On the other hand, biomass and used mainly for heat and electricity the current disposal methods for these agricultural generation, while second generation biofuels are residues have led to huge environmental issues. For obtained by highly-processed biomass and can be instance, straw burning results in atmospheric pollution employed in diverse industrial applications. Second

Green Chemistry Accepted Manuscript and affects human health13. generation biofuels are further divided into three sub- Plant-derived terpenes (or terpenoids) are the categories based on technologies and materials used for other attractive sustainable resource, which can be their production4. The common forms of liquid or gaseous considered as powerful platform for production of plant- biofuels are bio-liquids (including bio-alcohols such as based biofuels. Many such as Eucalyptus bioethanol and biomethanol), and biogas3,4 (Fig. polybractea, mints, eucalypts, pines and citrus produce 2).

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Table 1. Different generation and categories of biofuels biomethanol are the most common due View to Article suitable Online DOI: 10.1039/C8GC03860K Category/ Source/subs economic and technical potentials for internal combustion Product generation trate engines3,19. Firewood, wood Used as unprocessed chips, pellets, Bioethanol Primary form, mainly for animal waste, heating, cooking or Ethanol is a colorless, clear liquid with an agreeable odor forest and crop electricity purposes and pungent taste. Pure ethanol can be used as a vehicles residues fuel-like gasoline additive/petrol substitute to increase Bioethanol/ butanol (by fermentation of starchy or Seeds, grain octane number and improve the emissions released by sugar-rich crops), and sugars Biodiesel (by motor vehicles3,19. Due to the properties of ethanol, transesterification of plant oils) bioethanol is highly regarded as a renewable alternative for motor vehicles and transportation system. Bioethanol /butanol Consequently, it reduces the consumption of crude oil (using enzymatic 10,16,19 and decreases the adverse environmental impact hydrolysis), Methanol, Secondary Lignocellulosic 10 mixed alcohol and green by reduction of CO2 build up . biomass diesel (by thermochemical Direct use of bioethanol or in the form of a mixture processes) Biomethane with gasoline has a long history. Its usage was widespread (by anaerobic digestion) in the United States and Europe until the early 1900s. After the Second World War, the potential of bioethanol Biodiesel and bioethanol Algae, from algae and seaweeds, was largely ignored until the appearance of the oil crisis in seaweeds Hydrogen from microbes and green algae the 1970s. Since the 1980s, there was a growing interest regarding the use of bioethanol as a substitute fuel Bio-alcohols especially for transportation10. Brazil and the United

Alcohols are known as oxygenated fuels16. Each molecule States are the world's leading bioethanol producing

Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. of alcohol possesses a various number of oxygen atoms, countries with more than 80% of global production. The and the number of these atoms is inversely associated United States is the leading producer with an estimated with its heating value. In other words, the heating rate for production of more than 15,000 million gallons per year, the combustion stage decreases as the number of oxygen which accounts for more than half of global atoms increases. Practically speaking, any of the organic production19,20. Brazil is another major producer with an molecules of the alcohol family can be considered as a estimated production of more than 7,000 million gallons

biofuel. The prime examples of this family are butanol per year20. Fig. 3 depicts bioethanol production in Green Chemistry Accepted Manuscript 21 (C4H9OH), propanol (C3H7OH), ethanol (C2H5OH) and different countries around the world .

methanol (CH3OH) which are suitable for commercial Bioethanol can be produced from plentiful agricultural purposes10,17. Bioalcohols are defined as alcohols residues2 (Fig. 4). Bioethanol is also known as grain biologically obtained from renewable biomass sources3,18. alcohol since it is mostly made from the sugar Among all types of bioalcohols, bioethanol and components of plant materials and starchy crops9,19. It is

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generated by fermentation of the sugar components of bioethanol, and countries like Argentina are View considering Article Online DOI: 10.1039/C8GC03860K these substances19. Any kind of carbohydrates can be the possibility of corn as a source of biofuel in the used for bioethanol production3,10. These raw agricultural future1,6,10. wastes were generally categorized into two groups, which Three processing steps are required for bioethanol are a) sucrose-containing substances; and b) starchy production from sugar-rich crops: enzyme hydrolysis, crops3,10,19. fermentation, distillation/dehydration6,17,19. Hydrolysis of Eventually owing to the development of advanced carbohydrate by enzymatic treatment (also called technologies, lignocellulosic waste materials/cellulosic saccharification) is the initial step which releases sugars biomass such as wood and straw have also been added as from stored carbohydrate. This results in a fermentable suitable agro-wastes for economical production of sugar-containing solution19,20, which can be further bioethanol3,10,19. The primary examples of lignocellulosic hydrolyzed by yeast-derived invertase to release simple agro-wastes are rice straw, wheat straw, corn straw, and sugars, e.g., glucose and fructose (Scheme 1)3,6,11. This , in which cellulose is the chief component and step is followed by fermentation, during which simple which are available throughout the year1. Nevertheless, sugars are converted into ethanol by the action of bioethanol production from is Saccharomyces cerevisiae yeast (Scheme 2)3,11,20. more expensive than traditional starchy crops17, as the Distillation/dehydration, as the last step, is applied to the fermentation process of these cellulosic is fermented broth with the aim of recovery and more complex and longer19. concentration of ethanol. Distillation is an energy- The raw material used for bioethanol production is a consuming operation, which accounts for a significant crucial parameter for energy yield. For instance, part of bioethanol production cost20. The fermented broth and cellulosic bioethanol yield 9 times as much typically contains approximately 12% ethanol. The alcohol energy as the fossil energy used to produce them. It is can be purified up to 96% by distillation. also reported that bioethanol from corn yields 20-30% C12H22O11 (Sucrose) → C6H12O6 (Glucose) + C6H12O6 more energy than fossil fuel energy consumed to make (Fructose) (Scheme1) Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. it10. Among all agro-wastes, sugarcane juice and molasses C6H12O6→ 2C2H5OH + 2CO2 (Scheme 2) have been much exploited in recent years, yielding The process of bioethanol production from hydrated and anhydrous bioethanol. Brazil is one of the lignocellulosic materials is different. Lignocellulose is a biggest producers of sugarcane with 31% of global poly-carbohydrate complex which is composed of lignin, production. There are approximately 9 million hectares of cellulose, and hemicellulose. In this type of material, the sugarcane cultivated in Brazil. is another lignocellulose is first subjected to pre-treatment for

Green Chemistry Accepted Manuscript popular crop which is grown in many European countries delignification to release cellulose and hemicellulose and yields a higher amount of bioethanol than grains such before hydrolysis. The pre-treatment is performed to as wheat. The United States mainly uses cornstarch to break the matrix, decrease the degree of cellulose produce bioethanol, whereas Europe utilizes starch crystallinity and increase the fraction of amorphous obtained from wheat and . Canada also reported cellulose1. In fact, this step helps making lignocellulose plans for the significant future development of corn-based

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biomass more susceptible to further treatment such as , while it can be producedView Article fromOnline DOI: 10.1039/C8GC03860K hydrolysis with improved yield of monomeric sugars. The biomass by gasification process19. Production of type of pre-treatment can be physical (e.g., size reduction, biomethanol from biomass is environmental, economic pyrolysis, microwave heating and non-thermal and consumer benefit process17. It has been reported that irradiation), chemical (e.g., wet oxidation, acid or alkaline the total cost of methanol production from biomass is

treatments), physico-chemical (e.g., steam, ammonia fiber remarkably cheaper than its production from CO2.

or CO2 explosion) or biological (e.g., microbial treatment Furthermore, there is an increasing trend in methanol using white, brown and soft rot fungi)1 (Fig. 5). demand whereas the price of this fuel is expected to rise After pre-treatment step, enzymatic hydrolysis of in the future. Therefore, processing of biomass is the most cellulose and hemicellulose can be performed to produce cost-effective way to produce methanol9. For this reason, fermentable sugars such as glucose, arabinose, mannose, some countries such as Brazil and the US have paid much galactose, and xylose. In this stage, hydrolysis breaks attention to the production of biomethanol9. Moreover, down the glycosidic linkages to release pentoses and some other products such as syngas also can be produced hexoses. These hydrolyzed sugars can be then fermented from biomass. into bioethanol1,11. Lignocellulosic biomass is a valuable substance for the

Although most of the current studies reveals the production of methanol. It contains cellulose, potential use (by software simulation) of such hemicellulose, lignin and small amounts of proteins, , technologies that could reduce the environmental impact, and ash that can be decomposed to produce methanol 9 it is still necessary to evaluate in detail the processing biofuels . Biomethanol, especially from lignocellulosic cost, the purity of ethanol obtained from the different materials, has low emissions since the carbon content of plants, as well as the practical implementation of the alcohol is primarily derived from the carbon that was systems, these being the main relevant obstacles to sequestered in the growing of feedstock and is only being 2 establish energy saving technologies in the concentration re-released into atmosphere . It has been reported that of bioethanol22. Besides, during the process of obtaining sugar bagasse and corncob with the total Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. bioethanol, many waste and byproducts are generated, biomethanol content of 5.93% and 0.67%, respectively, 9 24 which need to be valorized since they can be reused to can be used as a promising source . Nakagawa et al. also obtain more bioethanol as well as being a source of other reported a high yield (55% by weight) for methanol valuable compounds. production from rice bran, whereas the yields for rice straw and husks were 36% and 39%, respectively. Apart Biomass-based Methanol from the sources above, other agricultural and animal

Methanol (CH3OH) is a simple organic liquid hydrogen Green Chemistry Accepted Manuscript biomass sources such as vegetable residues, wheat straw, carrier that acts as a hydrogen storage compound A9. It is butchery waste, fishery waste, and thinned wood have also known as wood alcohol since it was extracted from been introduced as potential materials for the biological wood as a co-product of charcoal. It is an alternative for production of methanol9. conventional motor fuels or a clean additive to the gasoline2,23. Methanol is mainly manufactured from non-

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25,26 CO . In the later stages, the methanol is derivedView Article from Online DOI: 10.1039/C8GC03860K Methanol is typically produced from the breakdown of the bio-oil through synthesis gas process5,25. - methyl esters or combination of ether with the methoxyl The process of pyrolysis can be classified into three groups9. So far, several processes have been introduced different categories named flash, fast and conventional for production of biomethanol, such as pyrolysis, bio- (Table 2)5. The categories differ in operating conditions synthesis, gasification, electrolysis and like process temperature, heating rate, solid residence photoelectrochemical methods. Each method has its own time, biomass particle size. The rate and extent of benefits/limitations and applications. The pyrolysis, as a decomposition during pyrolysis and distribution of conventional method, is particularly adapted for intermediate and final products are highly dependent on methanol production for diesel engines and gas turbine these effective parameters5,9. applications on a large scale, whereas electrolysis and Table 2. Classification of pyrolysis process and its photoelectrochemical methods, as new techniques, are products under different operating conditions still limited to lab scale. Bio-synthesis process is also used Type of pyrolysis as production method for gaseous fuels from a wide Conventional/slow Fast Flash range of biomass resources; however gasification is pyrolysis pyrolysis pyrolysis Operating considered as more preferable technique for the same conditions 9 Heating rate due to its cost-effective benefit . 0.1-1 10-200 >1000 (K/s) Pyrolysis is the first synthetic process was introduced Particle size 5-50 <1 <0.2 by Gulluetal in 19279. This method can produce biofuel (mm) Residence time 450-550 0.5-10 <0.5 with high fuel-to-feed ratios, and as a result, it has been (s) Temperature 850- 1050- attracting more attention than other production 550-950 (K) 1250 1300 methods5. The term “pyrolysis” is taken from the Greek words “pyro” meaning fire and “lysis” meaning Approx. product yield decomposition or cleavage into smaller constituent parts (%) Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. using thermal energy2,5. Oil 30 50 75 Char 35 20 12 The pyrolysis process of organic substances is very Gas 35 30 13 complex and consists of both simultaneous and successive reactions5. It involves a catalyzed reaction of hydrogen Gasification is another thermochemical processing and carbon monoxide under high temperature and method for synthesis of methanol from biomass. In this pressure9. In this process, the decomposition of

method, the biomass is initially gasified to produce Green Chemistry Accepted Manuscript components starts at 350 - 550 °C and rises to 700 - 800 intermediate product i.e., synthesis gas (syngas), which is °C in the absence of oxygen. The long chains of hydrogen, subsequently transformed into methanol under high oxygen, and carbon compounds break down into smaller pressure and temperature in a MeOH synthesiser5,27. parts in the form of gases, condensable vapors and solid - Production of syngas can be done through catalytic and 5,25-. The products of biomass pyrolysis are bio-oil (or bio- non-catalytic routes. The non-catalytic process requires crude), residual char and gases such as CH4, H2, CO2 and

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high temperature, while the catalytic route can be are used as gasifying agents. These conversionView processes Article Online DOI: 10.1039/C8GC03860K operated at lower temperature14. Fig. 6 is depicted the are typically suited for the wet biomass/feedstock. These production steps of biomethanol from carbohydrates processes may improve the overall process efficiency biomass by gasification and partial oxidation reaction3. As during conversion of biomass by increasing the contents

can be seen, the gasification involves reacting the biomass of products such as CH4, CO, CO2, H2, or other with oxygen or steam to decomposition the complex hydrocarbons26. carbohydrates substance and produce a gaseous mixture Biodiesel

consisting of H2 (22-32%), CO (28-36 %), CO2 (21-30 %) and Biodiesel is a clear amber-yellow liquid, which is

3 other hydrocarbons such as CH4 and C2H4 . The gases are chemically defined as mono-alkyl esters of vegetable oils further converted in a conventional steam- or animal fats. It is an interesting substitute to petro-fuel, reforming/water-gas shift reaction to predominantly which can be made from both edible and non-edible produce carbon monoxide and hydrogen (Scheme 3 & 4). oils11,28. Biodiesel has been probably received the most This step is then followed by high-pressure catalytic attention as a substitute fuel for diesel engines among all methanol synthesis as shown in Scheme 5 and 62. biofuels, due to its similar energy content and chemical structure28. It has the remarkable economic potential at Shift reactions industrial scale and has been commercially used in several CH4+H2O→ CO + 3H2 (Scheme 3) countries such as the United States, Brazil, Australia, CO + H2O →CO2 + H2O (Scheme 4) Malaysia as well as over European countries28,29. Methanol synthesis reaction Table 3 shows the major benefits of biodiesel over

8,11,28,30 CO+2H2→CH3OH (Scheme 5) conventional petrodiesel fuel . Thanks to these

CO2+3H2→CH3OH+H2O (Scheme 6) advantages, governmental policies are changing towards investment on research and production of biodiesel The gasification has advantages over other conversion particularly from crops with higher oil production. technologies. Some of these advantages are a) feasibility Considering the existing trend for biodiesel demand and Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. of use of any type of biomass (e.g., agricultural, forestry, the potentiality of increasing production, it is possible that chemical or organic wastes/by-products); b) feasibility of the production of biodiesel increases further in the near conversion of the entire carbon content of the biomass future. materials into fuel; c) the product gas can be converted Table 3. Major advantages and disadvantages of into a wide range of potential biofuels (e.g., methanol, as compared to petroleum diesel fuels

synthetic diesel, gasoline, H2 and Bio-Synthetic Natural Advantages

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Gas); and d) lower CO2 emission and high thermal Technical benefits Non-toxic, efficiency26. Non-flammable and non- explosive vapors Gasification process also can be performed in the form Perfectly miscible of hydrogasification or steam hydrogasification. Higher lubricity Hydrogasification uses hydrogen as the gasifying agent, Lesser flash point than petrodiesel whereas in steam hydrogasification, steam and hydrogen Synthesized from edible and non-

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edible oils Mauritia flexuosa, which are usable forView biodieselArticle Online DOI: 10.1039/C8GC03860K Better sulfur and aromatic contents production7. Safer handling and storage Moreover, some plant-based oils made from seeds have also been introduced to endow great potential for Environmental Environmentally friendly benefits making biodiesel such assal (Shorea robusta), neem

Reduction of CO2 in the atmosphere (Azadirachta indica), mahua (Mahua indica), besides Reduction of sulfur levels in the karanj (Pongamia pinnata) and ratanjyot ( atmosphere 6 Biodegradability curcas) . Non-edible vegetable oils, such as Karanja Renewability ( pinnata), , and Madhuca longifolia have also been reported as suitable seed oils to Economic benefits Job creation produce biodiesel28. Avoidance of urban migration Different parts of the fruits can be used for oil Provision of modern energy carriers to rural communities extraction. is extracted from the endosperm. Availability In oil palm, both mesocarp and seeds of the fruit are used, Energy security whereas, in peanut, castor bean, babassu, , , sunflower, physic nut, and cotton, the oil is Disadvantages Lower energy content Lower stability extracted from seeds. The oil content of each part in each Lower engine speed and power crop varies depending on species and anatomical Creation of engine durability differences. It has been reported that oil palm and physic problems and corrosion nut are the most advantageous biodiesel crops as they Creation of carbon deposition and polymerization in engine can produce approximately 8000 and 1500 kg of oil/ha, Since biodiesel is a product for the energy sector, oil respectively7. It is also obvious that the higher oil yield is for needs to be inexpensively corresponding to the lower production costs. Therefore, available in large quantities7. To increase the availability crops with high oil content are preferable28. Some crops Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. of oil, various vegetable oils and crops must be taken into such as soybean have a high value and consequently consideration. The idea to use vegetable oils for makes the production of a cost-effective fuel very renewable fuel competing with petroleum was proposed challenging. However, there are various types of low-cost since the beginning of the 1980s11. Nowadays, several oils and fats, such as animal-based restaurant waste crops have been put forward as a potential candidate for which can be converted to biodiesel11. The biodiesel can biodiesel production. Some examples are , also be made from other sources such as pork lard, beef

Green Chemistry Accepted Manuscript peanut, rapeseed, coconut, babassu, sunflower palm, tallow, and yellow grease. Processing these low-cost oils is castor bean, canola, corn, and cotton6,7,28. Fig. 7 shows usually challenging since the free fatty acids contents are the major oils used for biodiesel production in the United high in these oils and therefore cannot be converted into States in 201631. Also, there are some other palm species, biodiesel by an alkaline catalysis2,11. Another valuable for instance, Attalea maripa, Syagrus coronata, source of biodiesel is microalgae. The advantages of Astrocaryum aculeatum, Acrocomia aculeata, and microalgae as a feedstock for biodiesel production, over

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terrestrial plants, are that there is no requirement for soil therefore, it is considered as great View option Article Online for DOI: 10.1039/C8GC03860K fertility and, if marine algae are used, there is no need to commercially production of biofuel33. draw upon valuable and often scare supplies of To commercialize a biodiesel fuel on a large scale, freshwater29. The other main advantages as well as several determining factors such as costs of processing disadvantages of using microalgae for biofuel production and technology, transportation and storage of feedstock are shown in Table 432. and land use changes are involved33. In this respect, Singh and Gu, 201034 stated three requirements, which must be Table 4. The major advantages and disadvantages of fulfilled for a successful replacement of conventional fuel biofuel produced from microalgae by biofuel production process. These requirements are a) Advantages Disadvantages availability of sufficient sources for production at commercial scale; b) having standard specifications and High growth rate Low biomass concentration quality; and c) having a lesser finishing cost as compared to conventional fossil fuel34. In order to fulfil these Less water demand than Higher capital costs land crops requirements, more research studies are required to assess commercial viability of plant-oil resources, their High-efficiency CO2 mitigation economic efficiency, feasibility and modifications of technological process for commercialization of biodiesel More cost effective farming production33. As previously mentioned, herbal oil is the endless Although many plants resources and also primary source of biodiesel with a similar energetic technologies have been introduced for biodiesel content to diesel fuel11, which can be used as fuel for production, only few of them are economically viable and combustion engines after applying some modifications. can be implemented in commercial scale. One this Pure oils generally have a higher viscosity than diesel fuel handful resource is sativa, which is a fast- (approximately 10-20 times) and lower volatility11. Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. growing plant with high oil content (35–38%). Camelina- Therefore, the complete burning does not occur and based fuel has been in use for commercial and military consequently results in the formation of deposits in the aircrafts and it is a more efficient solution than fuel injector of diesel engines. The high viscosity of pure commercial biodiesel that absorbs water too easily33. vegetable oils (27.2 and 53.6 mm2/s) makes direct use of Another potential option for commercial biodiesel them impossible. To solve this issue, vegetable oils have production is Pennycress (Thlaspi arvense L.). It contents to be catalytically changed into biodiesel by

36% oil with high net energy output. A minimum amount Green Chemistry Accepted Manuscript transesterification or esterification process to reach a of 907 kg of this plant can be harvested per acre, which viscosity of 3.59 to 4.63 mm2/s7,11. allows for approximate production of 115 gallons of biodiesel. This plant is very short growing season and its Transesterification is the main conventional process to biodiesel properties is found to be excellent, and convert vegetable oil to their (m)ethyl esters in the presence of a catalyst11,29. Various esters such as methyl,

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ethyl, butyl, and 2-propyl can be obtained with the help of Apart from the transesterification method,View Article micro- Online DOI: 10.1039/C8GC03860K a catalyst, mainly potassium and/or sodium carbonate emulsification, thermal cracking and non-catalytic and alkaline metal alkoxides and hydroxides (sodium and supercritical methanol methods have been applied to potassium)2,11. Among them, alkaline metalalkoxides are pure vegetable oils, as reported by Yusuf et al.28 and generally preferred owing to their highest activity and Demirbas11. Regardless of the used method, the final high-yield production in a short reaction time2. In this biodiesel product should have physical properties close to process, triglyceride (oil) and alcohol react and those described by Demirbas11 (Table 5). consequently form methyl or ethyl-esters as the main

product and glycerol as a by-product (Fig. 8)35. Table 5. Physical characteristics of biodiesel Due to the high dependence of the transesterification Physical parameters Range process to the presence of a catalyst, these compounds Kinematic viscosity 2 3.3–5.2 have an important role in biomass transformation to range (mm /s, at 313 K) Density range (kg/m3, at 860–894 produce biofuels. Due to laborious preparation and high 288 K) cost, catalysts occupy a significant percentage of overall Boling-point range (K) >457 Flash-point range (K) 420–450 process cost, hence, the development of cost-effective Distillation range (K) 470–600 Vapor pressure (mm Hg, and stable catalysts to enhance the industrial production <5 at 295 K) of biofuels is essential for economic viability. In this Solubility Insoluble in water Stable, but avoid strong context, to reduce the required time and increase the Reactivity oxidizing agents efficiency of the reaction, other catalysts such as enzymes Light to dark yellow, clear Appearance, odor (e.g., lipases and esterase), acids (e.g., sulfuric and Light musty/soapy odor hydrochloric acids), and bases can be utilized7,8. The The are several factors affecting the (m)ethyl ester yield choice of a catalyst depends on quality and type of the efficiency and quality of biodiesel, like time and initial oil. For instance, the acidic oils require a basic temperature of incubation, the type of catalyst and its catalyst for neutralization of their free fatty acids

Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. concentration, the molar ratio of alcohol/vegetable oil, contents7. the purity of the reactants, nature, and composition of The second method to produce biodiesel is biomass and the methodology used11,28,29,35. The amount esterification. During esterification, free fatty acids react of oil content and its saturation level are critical factors in with low molecular weight alcohol such as ethanol or the quality of the final biodiesel product. The highly methanol, to produce ester (i.e., biodiesel) and water. Oils unsaturated fatty acids need to be modified by with high free fatty acid content, resulting from the hydrogenation since they increase polymerization risk in Green Chemistry Accepted Manuscript refining process of animal fats obtained from slaughter- engine oil and cause oxidative stability issues for fuel29. houses or oils extracted from sewage, are the prime Another important factor, which needs to be taken into examples of these acidic oils7. account, is the presence of water in oil and alcohol. Both of these two items must be anhydrous since the presence of water may lead to production from the existing

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free fatty acids. This undesirable by-product reduces the is also used as a prime source of energyView for Article cooking Online DOI: 10.1039/C8GC03860K efficiency of the process and complicates the purification and lighting. Cooking accounts for a considerable portion of glycerol7. of household energy consumption, especially in developing countries. Lighting is known as the second Biogas most common application for biogas, in particular in the

The digestion of bio-wastes under anaerobic condition areas where the electrical grid connection does not exist. results in the formation of a product in the gas phase, In these regions, biogas can be adapted for use in gas 39 which is called biogas11,36. It is a clean form of energy, mantle lamps . which is manufactured using a mix of anaerobic microbial Due to the significant advantages of biogas over other species, fermenting organic materials under controlled forms of gas, it is becoming a popular source of energy in conditions37. Biogas is a mixture of carbon dioxide, both developing and developed nations. The process methane, sulfur components, nitrogen, and hydrogen which is used for the production of biogas (anaerobic (Table 6). However, the main constituent (i.e., methane) digestion, AD) is considered the most energy-efficient and is an inflammable gas with no taste, color or odor36,37. The economical method although it has low carbon efficiency composition and yield in final methane varies depending and leads to large amounts of residues. It can drastically on the type of feedstock, conditions in the digestion reduce greenhouse gases and therefore is accounted as system and retention time38. an environmental treatment for recovery of clean energy from disposable residues38,40,41. It can also recycle plant nutrients and increase agricultural productivity37,39. Table 6. Composition of biogas Due to these applications and benefits, there is a great Composition Volume (%) interest in the production of biogas worldwide. For Methane 55-65 instance, in 2007, the biogas production in Europe Carbon dioxide 35-45 reached 6 million tons of oil equivalents with a yearly Hydrogen sulfide 0-1 Nitrogen 0-3 increase of more than 20%. Germany is the largest biogas Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. Hydrogen 0-1 producing country around the world and has the strong Oxygen 0-2 development of agricultural biogas plants on farms. It Ammonia 0-1 operates about 4,000 agricultural biogas production units Biogas, as an energy source, has many applications and on German farms opened in the last decade38. advantages. It is traditionally used for internal combustion Any type of biomass containing proteins, fats, engines to produce electricity and heat. However, its carbohydrates, hemicelluloses or cellulose as principal

potential use in fuel cells could increase its electric Green Chemistry Accepted Manuscript components can be used as biogas substrate38. This efficiency1,36,37,39. It can also be used as a fuel a) for water includes various raw materials such as sewage sludge, pumps and agricultural engines; b) for liquefied human excreta, animal manure, organic fraction of petroleum gas and gasoline engines37; c) for boilers1, municipal solid waste and the residues from crop and incubators and coolers37,38; d) for vehicle forest37,39. Algae could also be accounted to be a raw transportation38,39; and e) for heat generation36,39. The substance for production of biogas, which is gaining

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particular interest because some of them can largely grow wide range of energy crops that are grown in versatileView Article Online and DOI: 10.1039/C8GC03860K up without any oxygen supply requirement37. Only strong sustainable crop rotations36. lignified organic substances such as wood are not suitable As previously mentioned, anaerobic digestion is widely as biogas sources due to their slow anaerobic used for biogas production. This complex process is a decomposition38. It is annually estimated that about 1,680 biological process that converts the organic substance million dry metric tons of crop residues are produced in into energy-rich biogas under anaerobic conditions32,35. developing countries. This can be regarded as a significant This conversion is carried out by a particular ecosystem of portion of the source required for biogas production39. microorganisms through a series of metabolic stages, Among all agro-residues, food and food-processing wastes which is divided into four phases namely, hydrolysis, are the primary resources for this technology39. Food acidogenesis, acetogenesis and methanation38,39. In the waste is approximately composed of 25% and 42% of first step, the complex compounds containing domestic household and commercial waste, carbohydrates, lipids, and proteins are converted into respectively40. their soluble monomers/oligomers such as fatty acids, Anaerobic digestion of food waste is regarded as a amino acids, sugars or even glycerol by hydrolysis. highly suitable method compared to other thermo- This step is also called a liquefaction stage. This chemical bioconversion methods like gasification or process is facilitated by fermentative or hydrolytic combustion42. Surendra et al.39 stated that food waste is bacteria which release extracellular enzymes such as the best source for biomethane production due to the xylanase, cellobiase, cellulase, amylase, protease, and high amounts of moisture (>80%) and volatile solids (95% lipase38,39. Most of these bacteria are strict anaerobes of total solids). Food wastes are low in nitrogen content belonging to the genera Bifidobacteria, Clostridia, and (except meat waste) but rich in organic matter which is Bacterioides38. Afterwards, the process of acidogenesis is readily fermentable39. performed by acidogenic bacteria, which ferment the The proximate composition of food-derived residues soluble compounds. The output of this step is a mixture of can considerably vary depending on their original source. hydrogen, alcohol, carbon dioxide, and low molecular Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. Large ranges of moisture content (74–90%), volatile solids weight volatile fatty acids such as propionate and to total solids ratio (80–97%), and carbon to nitrogen ratio butyrate38,39. During the acetogenesis stage, alcohols and (14.7–36.4) are observed. There is a wide range of agro- volatile fatty acids are anaerobically oxidized by

substrates which can be used for the production of hydrogen-producing acetogenic bacteria into acetate, CO2

agricultural biogas, such as beet pulp, fruit, vegetable and H2.

pomace, silage, maize, sunflower, grass, and Acetate can also be formed from H2 and CO2 by

Green Chemistry Accepted Manuscript sudangrass36,43. The net energy yield per hectare is the hydrogen-oxidizing acetogenic bacteria. In the final stage, most important factor for choosing crops. The highest the groups of methanogenic strains produce a mixture of

gross energy belongs to maize and forage beets which methane and carbon dioxide from acetate, H2, and CO2. make them as a suitable ideal source of biogas38. Shortly, Only a few species can degrade acetate into methane and it is predicted that biogas production from energy crops carbon dioxide, such as Methanosarcina barkeri, will be increased and therefore, requires to be based on a Methanonococcus mazei, and Methanotrix soehngenii38,39.

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45 Fig. 9 shows the main steps of the bio-methanation low number of residues are required . At View the Article end Online of DOI: 10.1039/C8GC03860K process39. biomass separation, the particles are expected to be small The physical characteristics and chemical composition in size to increase reactive surface area, in order to of final biogas are highly dependent on several factors improve the productivity since the hydrolysis process is including the type of process, nature and physicochemical directly influenced by the porosity of lignocellulose-based properties of the organic substance, operation conditions biomass.

(pH, temperature, carbon/nitrogen ratio, retention time) Fractionation techniques have been employed in 36–38,42 and origin of the substrates . These parameters biofuel production for decades, but new improvements strongly affect the design, performance, and stability of are highly required. Recently, currently available the digestion process and must be set up within a fractionation technologies are significantly improved and 36,42 desirable range for an efficient production . Any drastic emerged as an effective way to minimize overall cost and change in controlled condition for operation can adversely increase the separation yield of lignocellulose. Different affect the biogas production. For instance, carbohydrates hybrid fractionation techniques are employed for the and proteins have a faster conversion rate compared to pretreatment of biomass for biofuel production. other components. However, they yield a lower quantity Dry fractionation of biogas. Fat provides the highest yield; however, due to Novel dry fractionation processes were recently shown to its poor bioavailability, it has a longer retention time. - have significant advantages by decreasing the use of The carbon/nitrogen ratio in the substance used must be water, solvents, and chemical reagents as well as meeting well balanced (between 15 and 30) in order to avoid other principal requirements for more efficient biofuel ammonia accumulation during processing38. The time and production. These separation techniques are essential to frequency of harvest are also considered as effective generate biomass with more appropriate composition and parameters which notably affect biogas quality and its an increased rate of accessibility by enzymes or final yield38. microorganisms during further fermentation steps. The

Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. use of this processes in combination with other processes Innovative technologies to improve the production of result in a more efficient fractionation. Chuetor et al. biofuels (2015)46 separately combined ultrafine milling with turbo- Novel fractionation technologies fractionation (size and density-dependent) and with

Lignocellulose-based biomass has considerable potential electrostatic fractionation technologies in order to as a raw material in biofuel production44 and its produce fractions from rice straw to be employed in the

separation into cellulose, hemicellulose, and lignin is one bioethanol industry. The specific energy requirement of Green Chemistry Accepted Manuscript of the most crucial steps of this production. both techniques to reduce particle size was between 12.5 Unfortunately, expensive, wasteful and energy-consuming and 22.4 Whkg-1, which indicates that energy pretreatment processes are still employed. Therefore, consumption was almost negligible compared to other novel technologies that allow a more efficient separation conventional techniques (i.e., knife and ball milling or with low resource consumption and the generation of a thermal treatments using stream). The processing time

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50 was considerably shortened, and fractions presented a George et al. 2015 designed a series ofView protic Article Online ILs DOI: 10.1039/C8GC03860K proper structure, size, and composition. Compared to containing hydrogen sulfate anion. The developed ILs untreated biomass, the glucose yield, and ethanol could enhance enzymatic saccharification yield, and production during 72 h of fermentation was increased by triethylammonium hydrogen sulfate was the most 83-103% and 75-95%, respectively. efficient IL at increasing digestibility of the biomass, while Piriou et al. 201847 developed an efficient dry providing better thermal stability with less residual fractionation process for separation of lignocellulosic generation. Most interestingly, due to their efficiency and biomass. The fragmentation of the biomass was low cost, some of the tested ILs could be replaced with conducted using a vibrating mill and a rotary ball mill. industrially-used chemicals like ammonium hydroxide After fragmentation, an additional step of triboelectric solution. Brandt-Talbot et al. (2017)51 also tested static charging was employed in a dynamic fluidized air triethylammonium hydrogen sulfate ($1 kg−1) to bed for the deviation of the path in the electric field of the fractionate the grass x giganteus into a charged particles in order to sort them efficiently. The cellulose-rich pulp and lignin. With IL treatment, sorted particles were collected since they were attached enzymatic saccharification of the pulp could lead to the to the electrode. In general, dry processes are good release of 77% of the glucose from the biomass. Besides candidates for biomass fractionation since the excess use high sugar yields, ILs could be repeatedly used (up to 4 of water is eliminated. However, novel, efficiently- times, with 99% recovery each time).

developed ionic liquids with low cost have also been used The efficiency of ILs depends on the biomass to be recently for the fractionation of biomass. fractionalized since each IL presents a different chemical Novel ionic liquids affinity to a different biomass, hence, ILs should be

Ionic liquids have been used in biofuel production for carefully designed to show effectiveness against a varied 52 decades. Nevertheless, their production methods and type of biomass. In this context, An et al. 2015 process yield became less effective and favorable for a developed cholinium ILs to be effective for fractionation of grass lignocelluloses and eucalyptus biomass and Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. biofuel production with expected productivity levels especially for industrial scale. Due to the recent advances obtained a glucose yield of 58-75%, however, the same IL 52 in chemical sciences, previously-used ionic liquids (ILs) was inefficient for pine biomass . Moreover, ILs could be were recently replaced with low-cost ILs as green solvents recycled 8 times with total recovery of 75%. have been employed for the pretreatment of In some cases, standalone pretreatment of biomass lignocellulosic biomass48. The ILs are mostly designed at using IL is not efficient and its efficiency can be enhanced

low cost, and for lignocellulose delignification, it is by combining with alkali-based treatments. Heggset et al. Green Chemistry Accepted Manuscript essential to avoid carboxyl, hydroxyl, and aromatic groups 201653 compared the efficiency of 1-ethyl-3- in the structure since the delignification capacity, and pKa methylimidazolium acetate (EMIM-OAc) as an IL (100 oC values of the conjugate acids of the anions are linearly for 6 h) and alkali-based treatment (NaOH/urea) (-18 °C correlated49. for 24 h) for Norway Spruce chips fractionation. Both methods could enhance the enzymatic digestibility of

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58 glucan and mannan in the biomass compared to 2018 developed a novel hybrid organosolvView technique Article Online DOI: 10.1039/C8GC03860K untreated material. Interestingly, combining the two combined with stream explosion method for fractionation methods increased monosugar yield between 20-50%. of birch biomass. Employing explosive discharge at the Similarly, Nargotra et al. 201854 combined IL (ionic liquid end of process, pretreated solids presented high cellulose 1-butyl-3-methyl imidazolium chloride) with alkali (77.9% w/w) and low lignin (7% w/w) content. The treatment (NaOH). The enzymatic digestibility of ethanol concentration obtained in this study was claimed sunflower stalk biomass was significantly enhanced and to be the highest in literature for birch bioprocess. the combination of two treatments resulted in a higher However, it is essential to adapt this technique to produce sugar yield (163.42 mg/g biomass) than only IL treatment ethanol using other biomass. In this context, Matsakas et (79.6 mg/g biomass). al. 201959 adapted this technique on spruce biomass and

Organosolv processes similarly, they obtained the highest level of ethanol reported for spruce. Organosolv processes was found to be effective for the fractionation of lignocellulosic biomass and has been used Microfluidic technology in biofuel production. Suriyachai et al.55 developed a one- Microfluidic platforms (lab-on-a-chip concept, micro- step formic-acid catalyzed organosolv process for reactors) have been utilized to elucidate different sugarcane bagasse fractionation. A glucose recovery of biological phenomena. Recently, microfluidic technology 84.5% was obtained while the fractionalized biomass has proven to be an interesting tool for the biofuel showed a decreased crystallinity. Kubota et al.56 employed industry since it allows the manipulation of biofuel- an organosolv process for and producer microorganisms as well as essential molecules could obtain cellulose-enriched fibers (fibers containing used in biofuel technology60. Microfluidic technology can 78% cellulose) without using any toxic solvents. Grande et be used in the whole process of biofuel development. One al. 201557 developed an OrganoCat process consisting of a of the principal advantages of this platform came forward biphasic system containing water, solvent (2-methyl with the development of microchip-based electrophoretic

Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. tetrahydrofuran (2-MeTHF)) and catalyst (oxalic acid). sequencing. With this system, biomass at 100 gL-1 could be Since this system provides faster processing times and fractionated in 3 h without formation of by-products and reduced reagent consumption, it is convenient to obtain water, and organic phase could efficiency be recycled 4 next-generation biofuel producer microorganisms for times leading to an economic advantage over other further use in either academic research or in industrial methods. productions61. Pacheco et al. 201362 developed a 96-well

Although these processes are well-established and microplate as a microreactor platform for microalgae Green Chemistry Accepted Manuscript excellent for delignification, they usually offer poor screening. This simply-designed system presented biomass deconstruction. Currently, organosolv processes essential data for the optimization studies of some basic don’t meet specific requirements for industrial biofuel growth kinetics of microalgae used in biofuel production, 63 productions and combination with other methods to while it allows substrate and space savings. Seguel develop hybrid models is highly required. Matsakas et al. manufactured a 3D-printed microfluidic device for

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microalgae Dunaliella sp. growth kinetics. Although the biodiesel production. During transesterification,View theArticle mass Online DOI: 10.1039/C8GC03860K microdevice did not meet the expectations, it was transfer was significantly improved, and it took only 10 possible to analyze important parameters such as min to have an accurate fatty acid analysis with a reduced microalgae damage test, surface retention, cell density, microalgal cell (a few milligrams).

CO2 and nutrition solubility as well as specific growth rate. Microfluidic platforms are excellent candidates to

Catalyst optimization in microfluidic systems replace conventional benchtop methods that are mostly laborious and time-consuming. Lim et al. 201467, Microfluidic technology is also a great tool for rapid manufactured an integrated microfluidic system catalyst optimization in biofuel production. Zhou and consisting of microchannels, micropillar array, cell Lawal, 201464 developed a microreactor system, chamber, output reservoir, and filtration unit in order to mimicking monolithic reactor, for green diesel production perform essential analyses such as microalgae culture, from hydrodeoxygenation of microalgae (Nannochloropsis accumulation and extraction for biofuel production in salina) oil, using NiMo/γ-Al2O catalyst. The microreactor a single device. Lipid extraction efficiency was 13.6% system allowed a proper mass transfer as well as good higher than the Bligh-Dyer method with less isopropanol yield of hydrocarbon and microalgae oil conversion rate. use comparing to the conventional method. Wang et al. Increasing hydrogen to oil ratio (1000 SmL/mL), residence 201668 conducted the synthesis of triglycerides from time (1 s), temperature (360 oC), and pressure (500 psig microalgae oil in a microreactor system packed with H2) could enhance catalyst activity. C13 and C20 immobilized lipase. Compared to the batch reactor, they hydrocarbon yield of 56.2%, carbon yield of 62.7% were obtained a significant reduction (87.5%) in reaction time obtained together with almost total microalgae oil with 2.25-fold more lipase reuse time. The adaptation of conversion (98.7%). They used the same microreactor this bioconversion technology to different biofuel system for biodiesel production65 while comparing three production has high potential to be a cost-effective catalysts (1% Pt/Al2O3, 0.5% Rh/Al2O3 and presulfided approach in the biofuel industry. NiMo/Al2O3). They evaluated the principal conditions Droplet-based microfluidics Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. that lead to coke formation during the catalytic treatment of biomass, which causes significant catalyst deactivation Microdroplets generated in a microfluidic platform also and found that accumulation of coke decreased in the bring a great advantage in the rapid and cheap analysis of order NiMo> Pt > Rh. These studies showed that several parameters influencing biofuel production. Large- microreactor systems provide a rapid and efficient scale biofuel production is performed via fermentation of catalyst system for biofuel production. Transesterification sugars from plant biomass, nevertheless, recently, biofuel

processes are common in biodiesel produced using production from photosynthetic organisms have drawn Green Chemistry Accepted Manuscript microalgal biomass. significant attention. Hence, it is important to select Conventional transesterification processes are appropriate organisms that give the highest yield. inefficient for obtaining a good quality fatty acid. In this context, Abalde-Cela et al. 201569 developed a Therefore, Liu et al. 201866 developed a microreactor for microdroplet system involving encapsulation of the rapid analysis of fatty acid profiles for continuous genetically-modified cyanobacteria in droplets, pico-

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injection of components into the system and fluorescence engineering also allowed the discovery of newView methods Article Online to DOI: 10.1039/C8GC03860K detection along the microchannel. It was possible to carry manipulate lignin composition73. Nevertheless, lignin out simultaneous screening of strains with different levels engineering is a great challenge since the applied of ethanol production. Similarly, Sung et al. 201670 technique can cause a significant loss in integrity in designed a PDMS-based microfluidic device for microalgal vessels as well as tissues that are responsible for the

74 75 cell (Chlorella vulgaris) growth and CO2 transfer into each transportation of water and nutrients . Yang et al. 2013 microdroplet for the bioconversion of biomass by developed a new approach that decreases lignin content microalgal cell were significantly enhanced and comparing while maintaining the structure of vessels by to flask culture, the cell growth was improved. More overexpressing of transcription factors in native tissues. recently, Li et al. 201871 produced a microfluidic platform This strategy allowed the reduction of lignin content to produce gelatin hydrogel microdroplets for high- and enhanced the polysaccharide deposition and throughput sorting of microalgal clones. The system consequently resulted in higher sugar yields for further allowed the growth of cells, metabolite production, enzymatic treatments. Smith et al. 201376 successfully selection of microalgae with high metabolite production designed a new miRNA to reduce lignin biosynthesis by used in biofuels, and cell recovery. silencing CCR1 (cinnamoyl-CoA reductase 1) using

Genetic/metabolic engineering pAtCesA7 promoter without disturbing vessel integrity. In a more recent approach, Eudes et al. 201577 altered the Genetic engineering of plants Shikimate and phenylpropanoid pathways to reduce the Conversion of cellulosic biomass as a renewable source is availability of metabolites that play key roles in lignin an essential step for biofuel production. However, due to production pathway. Lignin modification carries great the several limiting factors, these processes can be of high importance since it is associated with the pretreatment cost. Therefore, new techniques to reduce the number of need for biofuel production. steps required in pretreatment processes while increasing Altering wall sugar component the yield and decreasing overall cost are very necessary72.

Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. The application of genetic engineering in plants is shown Plant cell wall mainly consists of carbohydrates, and a to be an effective method and has been employed with considerable amount of pentose sugars are present in the increased efficiency over the years. Most of the plant wall, and these pentose sugars are difficult to be biomass consists of cell walls, and the content and fermented efficiently. Hence, new approaches to properties of this biomass are the main factors to reach decrease the pentose level in the wall is essential. Altering an economically-viable production with increased nucleotide sugar conversion pathways can be another 73 productivity. alternative method to increase hexose/pentose ratio . Green Chemistry Accepted Manuscript Rautengarten et al. 201478 used a new technique to Lignin reduction characterize six bifunctional UDP-rhamnose (Rha)/UDP- The lignin biosynthesis pathway has been well-studied, galactose (Gal) transporters from Arabidopsis in order to and the modification of lignin structure has been identify important alterations during the biosynthesis of investigated in the last few years to improve saccharification yield. New advances in genetic

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Rha- and Gal-containing sugars, which resulted in Biofuel obtained from microalgae was View proven Article Online to DOI: 10.1039/C8GC03860K increased β-1,4 galactan deposition. significantly reduce the amount of CO2and sulfur

83 Inhibition of endogenous pathways emissions compared to conventional biomass . Due to the great advantage of microalgae over other biosources, Besides carbohydrates, lignocellulosic biomass contains a including plants, genetic modifications are important tools significant amount of acetyl and methyl esters, which can to enhance the quality and productivity of next- block the access of some enzymes to access to generation biofuels. Over-expressing some genes in order polysaccharides. Furthermore, these esters were found to to alter specific metabolic pathways in microalgae for present inhibitory effect on further fermentation enhanced biofuel yield can be achieved using genetic and processes during biofuel production79. Genetic metabolic engineering. It has been reported that engineering has been used to reduce lignocellulosic acetyl triggering triacylglycerol (TAG) accumulation in groups by altering the biosynthesis of acetylated microalgae can significantly benefit biofuel production. polysaccharides80. Studies performed with Arabidopsis Kaye et al. 201584 could enhance the biosynthesis of revealed that downregulation of genes encoding proteins polyunsaturated fatty acids in Nannochloropsis oceanica, involved in Reduced Wall Acetylation process could which is a great candidate in the biofuel industry, by decrease the acetylation levels by 25%81. Unfortunately, overexpression of endogenous Δ12 desaturase (NoD12). the investigations are limited to Arabidopsis and new This overexpression using native genes and promoters studies with other plants are required to employ these significantly enhanced conversion of these fatty acids in genetic engineering techniques for large-scale biofuel the TAG. Kamennaya et al. 201585 engineered the production. cyanobacterium Synechocystis sp. PCC6803 to increase Biomass increase the number of copies of the endogenous bicarbonate Overall plant biomass can be increased with genetic transporter BicA, which is required for a more efficient engineering, more specifically by modification of plant CO2use. Under CO2 pressure, this modified strain was able growth regulators. According to the studies with to produce additional BicA, which resulted in a biomass Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. transgenic trees, increased gibberellin biosynthesis by and growth rate twice more than the wild-type. Chien et overexpressing a responsible regulatory gene was found al. 201586 genetically engineered Chlorella sp. by codon- to provoke biomass growth, resulting in more biomass per optimization of several genes. The expression of genes 82 unit area . Different genetic engineering techniques have encoding enzymes of the biosynthetic Kennedy pathway, also been employed in order to improve some factors which is a metabolic pathway for the production of TAG,

(i.e., carbon allocation, uptake of CO2, N2 and other resulted in increased TAG levels (20-46 wt%) and total

Green Chemistry Accepted Manuscript essential sources, efficient utilization of O2, water, and lipid storage (35-60 wt%) compared to the wild-type. The other nutrients, respiration, and even circadian clock) to malic enzyme has a critical role in pyruvate metabolism 72 increase the overall biomass . and carbon fixation in microalgae. Xue et al. 201587 Genetic engineering of microalgae overexpressed the gene encoding malic enzyme in Phaeodactylum tricornutum and obtained an increase in malic enzyme activity. Malic enzyme overproduction

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92 significantly increased total lipid content (2.5-fold) while systems as potent and programmable antimicrobialsView Article Online, DOI: 10.1039/C8GC03860K the growth rate was maintained. The cell shape of designing the vaccination of microorganisms against microalgae became thicker and shorter, indicating a high- invasive genetic elements93 and controlling gene loaded oil inside. expression in an inducible and reversible way94,95.

CRISPR/Cas9 gene editing machinery Microalgae-based bioresources are considered the third-generation biofuel feedstocks and genome editing CRISPR/Cas9 technology applied to algae tools like CRISPR/Cas 9 are important candidates to The discovery of CRISPR (interspaced short produce next-generation biofuels. Due to the novelty of palindromic recurrence grouped regularly) / Cas9 the technique, there is a limiting number of studies (nuclease 9 associated with CRISPR) has significantly performed with genome editing of microalgae using changed the field of genome engineering and paved the CRISPR tool. Although genome editing has been well way for a wide variety of applications of different established in some organisms, the application in industrial branches88. CRISPR-Cas9-mediated genome microalgae was shown to be a challenging process. The editing has emerged as a novel tool in genetic engineering first study with CRISPR/Cas9 system was conducted in to improve essential traits in microorganisms to make the Chlamydomonas reinhardtii96. In this study, the transient product viable for industrial applications. expression of Cas9 and single guide RNA genes was The CRISPR/Cas 9 technology is based on the genome successfully carried out. edition, allowing to insert, eliminate or alter a desired However, Cas9 toxicity was observed when Cas9 was genetic material in specific places of the genome. This produced constitutively in microalgae. For being the first system consists of two essential molecules: i) the Cas9 study in the application of CRISPR/Cas9 in microalgae, endonuclease DNA and ii) a single guide RNA (gRNA). effective methods were required for proper gene editing. While the previous molecule acts as a pair of "molecular After this study, Shin et al. 201697 employed this powerful scissors" that unwind and consequently cut the target tool in Chlamydomonas reinhardtii. The induced DNA at specific loci, the gRNA has 20 bases long to make mutations were obtained at three different loci (MAA7, Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. sure that the desired part of the DNA is being attacked89. CpSRP43, and ChlM) and mutagenic efficiency was The Cas9 RNA-guided enzyme originated from the CRISPR- enhanced up to 100-fold comparing to the previous study. Cas adaptive bacterial immune system and is transforming The improvement of the knockout effectiveness of Cas9 the science of molecular biology by providing an advanced ribonucleoproteins could pave the way for the new genomic engineering tool. industrial applications of microalgae for biofuel This technique is based on the principles of Watson- production. Wang et al. 201698 also engineered the

Crick base pairing and was adopted in some laboratories Green Chemistry Accepted Manuscript genome of model microalgae Nannochloropsis spp. by and fields due to its diverse applicability88,90. Recent CRISPR/Cas9 using nitrate reductase. The isolated applications of CRISPR/Cas9 are creating new mutants could maintain metabolic activities normally opportunities to investigate the function of genes and under NH4Cl but could not survive under NaNO3. reveal important biological knowledge such as microbial CRISPR/Cas9 technology applied to crops consortium engineering91, establishing CRISPR-Cas9

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CRISPR/Cas9 technology has also been employed for omics tools have been used to evaluate the perturbationsView Article Online DOI: 10.1039/C8GC03860K crops to increase biofuel production. Park et al. 201799 resulting from the application of variable biotic and applied CRISPR/Cas9 to the switchgrass (Panicum abiotic factors (temperature, sludge retention time and virgatum), as an important crop for production, organic loading rate) to the system103–105. Applied to by targeting an essential enzyme that involves in the algae, the omics approach is seen as an opportunity to biosynthesis of monolignol. Among three tested 4- define control points governing metabolic flux, and to Coumarate: coenzyme A ligase (4CL) genes, Pv4CL3 was propose rational algal strain-engineering targets106,107.

selected for CRISPR/Cas9 treatment due to its Microbial tolerance during biofuel production overexpression in lignified stem tissues of the plant. Biocatalysts have been widely used for biofuel Among 39 generated transgenic plants, four plants production since they can efficiently degrade presented tetra-allelic mutations, and the knockout of heterogeneous polymers into simpler form while allowing Pv4CL1 caused a reduction in cell wall thickness (8-30% the fermentation occurs simultaneously to produce reduction in lignin, 7-11% increase in glucose release, 23- biofuel. However, microbial tolerance against increased 32% increase in xylose release). This study was essential final product concentration is usually limited since for the further application of CRISPR/Cas9 technique in biofuels108, as natural antimicrobials, can disrupt the plants to improve biofuel production. cellular macromolecules, hence, the techniques to avoid Omics technologies chaotropic effect on biocatalysts caused by final product A way to get around the difficulty related to lignocellulosic should be employed for a continuous biofuel material bioconversion has recently been explored production109.

thought omics approaches. Co-cultures of bacteria which As a good example, fermentation with Clostridia is used to can directly ferment lignocellulosic biomass have been produce biobutanol together with acetone and reported to display increased rates of cellulose hydrolysis ethanol110–112. Before fermentation, pretreatments of and higher ethanol titers than observed in monocultures. lignocellulosic materials are required to produce the

Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. To this purpose, metagenomics, transcriptomics, highest possible fermentable sugars from lignocelluloses proteomics and metabolomics are useful tools to better with a minimal risk of contamination by inhibitors. understand microbial communities, enzyme interactions, Thereafter, cellulolytic enzymes convert the substrates 100,101 and how lignocellulose breakdown occurs . The into a fermentable hydrolysate. The improvement of the establishment of microbial consortia in naturally yield of fermentation is limited by the tolerance of degrading lignocellulosic compound ecosystems has Clostridia to butanol113,114. For the latter, strain

proven its value to propose synthetic microbial engineering to obtain a hyper-butanol producer is being Green Chemistry Accepted Manuscript ecosystems with genetic content related to a desirable set investigated. of biochemical functions. Comprehensive and consistent One of the most common method is the product knowledge of a biological system, and of the interactions removal via different separation techniques to recover which occur in, is a first required step to conceive highly-purified biofuel115. Comparing to conventional 102 synthetic biological systems . For biogas production, batch processes, new separation systems integrated with

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fermentation process that allows in situ recovery are consequently, new strains with improved toleranceView Article Online to a DOI: 10.1039/C8GC03860K shown to result in a 25-times more biofuel production in wide range of alcohols can be selectively produced114. 21 days of continuous process116. Beside final product inhibition, pretreatment of lignocellulosic biomass could Conclusions also generate by-products that show inhibitory effect on Plant/agricultural materials in particular low value/waste biocatalysts. Salvachúa et al. 2011117 showed that biomass present great potential for production of various inoculation of white-rot fungi could decrease the types of biofuels including bioethanol, biomethanol, inhibitory effect of by-products and allow the complete biodiesel and biogas. They have superiority in terms of fermentation of glucose into ethanol using Saccharomyces environmental effects, economic potential and cerevisiae. sustainability as compared to other fuel resources. Immobilization techniques have been widely used for Therefore, the industry sectors have been shown decades to provide an additional protection to growing interest to utilize such agro-waste residues. biocatalysts during fermentation118. Encapsulation of Although various biomass sources are introduced for biocatalysts using polymeric matrices were also found to biofuel production (e.g., food, livestock, forestry, fishery be effective to decrease or completely eliminate the and plantation), the choice of biomass type for production inhibition caused by final biofuel. As previously of biofuel is crucial. Starchy/sucrose-containing crops are mentioned, biofuels show chaotropic activity against considered as most suitable source for bioethanol, biocatalysts, therefore, immobilization material should be whereas, polycarbohydrate complexes such as kosmotropic (order-making) in order to stabilize lignocellulosic materials are less popular due to their macromolecular systems of the used biocatalysts. In this complex processing. For biodiesel production, oil seeds context, hydrophilic polymers such as agarose, calcium- biomass has preference due to their high energy yield, alginate conjugate, and carrageenan can be used to while in case of biogas, food-processing waste are chosen encapsulate biocatalysts108. Liu et al. 2014110 immobilized as best source of biomass in this regard. However, there Clostridium acetobutylicum using adsorption technique on Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. is a need to optimize the processing conditions according a novel macroporous resin, KA-I, to produce biobutanol. to the matrix used as well as the different stages of the Biocatalysts showed improved butanol tolerance and high process, with distillation being a key step. There is a need fermentation yield. Immobilization could allow the use of for innovative strategies to make the process more biocatalysts repeatedly for the continuous production of efficient. In this sense, new energy saving strategies have biofuel. been used, which have shown promising results as

Beside these techniques, genetic and metabolic alternatives to conventional distillation to obtain ethanol, Green Chemistry Accepted Manuscript engineering techniques can be employed to create more either zeotropic or anhydrous, from fermented broths. robust strains with high tolerance against inhibitions. Additional research should focus on the development of Moreover, it is possible to identify the main factors that economically viable energy-saving distillation systems, the contribute to the final product inhibition and impact of processing variables on bioactive extraction, the

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expansion of such operations and the characterization of 15. View Article Online 19. Yatri R. Shah, Dhrubo Jyoti Sen. DOI:Int J10.1039/C8GC03860K Curr Sci Res. bioactive compounds and related biological benefits. 2011;1:57–62. 20. Gavahian M, Munekata PES, I, Lorenzo JM, Conflicts of interest Eş Mousavi Khaneghah A, Barba FJ. Green Chem. 2019; There are no conflicts to declare. 21. Gallo JMR, Bueno JMC, Schuchardt U, Gallo JMR, Bueno JMC, Schuchardt U. J Braz Chem Soc. 2014;25(12):2229–43. 22. Dahiya S, Kumar AN, Shanthi Sravan J, Chatterjee S, Acknowledgements Sarkar O, Mohan SV. Bioresour Technol. 2018;248:2– 12. Francisco J. Barba would like to acknowledge the 23. Demirbas A. Prog Energy Combust Sci. 2005;31(5– support from “Generalitat Valenciana” through the 6):466–87. 24. Nakagawa H, Harada T, Ichinose T, Takeno K, project for emerging research groups GV/2018//040. Matsumoto S, Kobayashi M, et al. Japan Agric Res Q José M. Lorenzo is members of the MARCARNE JARQ. 2007;41(2):173–80. 25. Lücking L. 2017. network, funded by CYTED (ref.116RT0503). 26. Park CS, Roy PS, Kim SH. In: Yongseung Yun, editor. Gasification for Low-grade Feedstock. InTechOpen; 2018. References 27. Demirbas A. Energy Sources, Part A Recover Util 1. Sarkar N, Ghosh SK, Bannerjee S, Aikat K. Renew Environ Eff. 2008;30(6):565–72. Energy. 2012;37(1):19–27. 28. Yusuf NNAN, Kamarudin SK, Yaakub Z. Energy 2. Demirbas A. Prog Energy Combust Sci. 2007;33(1):1– Convers Manag. 2011;52(7):2741–51. 18. 29. Williams PJ le B, Laurens LML. Energy Environ Sci. 3. Demirbas A. Energy Convers Manag. 2010;3(5):554. 2008;49(8):2106–16. 30. Atabani AE, Silitonga AS, Badruddin IA, Mahlia TMI, 4. Nigam PS, Singh A. Prog Energy Combust Sci. Masjuki HH, Mekhilef S. Renew Sustain Energy Rev. 2011;37(1):52–68. 2012;16(4):2070–93. 5. Jahirul M, Rasul M, Chowdhury A, Ashwath N. 31. 2017. Energies. 2012;5(12):4952–5001. 32. Demirbas MF. Appl Energy. 2011;88(10):3473–80. 6. Kumar A, Sharma S, Vasudevan P. J Sci Ind Res. 33. Ziolkowska JR. Biotechnol Reports. 2014;4:94–8. 2005;64:822–31. 34. Singh J, Gu S. Renew Sustain Energy Rev. 7. Bergmann J., Tupinambá D., Costa OY., Almeida JR., 2010;14(9):2596–610. Barreto C., Quirino B. Renew Sustain Energy Rev. 35. Schuchardt U, Sercheli R, Vargas RM. J Braz Chem 2013;21:411–20. Soc. 1998;9(3):199–210. 8. Ak N. Life Sci J. 2013;10(7s):1097–8135. 36. Balat M, Balat H. Energy Sources, Part A Recover Util Published on 17 April 2019. Downloaded by University of York 4/17/2019 3:27:41 PM. 9. Shamsul NS, Kamarudin SK, Rahman NA, Kofli NT. Environ Eff. 2009;31(14):1280–93. Renew Sustain Energy Rev. 2014;33:578–88. 37. Martins das Neves LC, Converti A, Vessoni Penna TC. 10. Balat M. Energy Convers Manag. 2011;52(2):858–75. Chem Eng Technol. 2009;32(8):1147–53. 11. Demirbas A. Energy Sources, Part B Econ Planning, 38. Weiland P. Appl Microbiol Biotechnol. Policy. 2008;3(2):177–85. 2010;85(4):849–60. 12. Demirbas A, Arin G. Energy Sources. 2002;24(5):471– 39. Surendra KC, Takara D, Hashimoto AG, Khanal SK. 82. Renew Sustain Energy Rev. 2014;31:846–59. 13. Demirbas A. Energy Sources, Part A Recover Util 40. Browne JD, Murphy JD. Appl Energy. 2013;104:170– Environ Eff. 2009;31(17):1573–82. 7. 14. Naik SN, Goud V V., Rout PK, Dalai AK. Renew Sustain 41. Chaiprasert P. J Sustain Energy Environ Spec Issue.

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